Check at an auto-parts store such as advance auto; quite a few of them carry underglow neon kits that have longer tubes than those for PC. the underglow kits would also use 12VDC, as said before you can easily get a wall adapter for that.

As said above you will probably only be able to power about 2 cathodes per power adapter.

Just notice i got my wires swapped, take the black (+) wire coming from the transformer, and that should go to the yellow wire on the molex connector. And the white, or whatever color wire it is (-) from the transformer and connect that to the black wire from the molex.